I'll test that out today and see if that works...

Also, check out the post about "providerPath" in this thread: http://www.dotnetnuke.com/Community/Forums/tabid/795/forumid/90/threadid/128403/scope/posts/Default.aspx#131192

I do have that entry in my web.config, but that path does not exist.  Nor does it exist in the install download.

Well, I went through the event log, trying to figure out each exception and remove the cause.  So far I've been able to remove most of them, and aside from the exceptions thrown by Googlebot searching, I only have one commonly recurring error:

AssemblyVersion: 04.05.01
PortalID: -1
PortalName:
UserID: -1
UserName:
ActiveTabID: -1
ActiveTabName:
RawURL: /Admin/Security/WindowsSignIn.aspx
AbsoluteURL: /Admin/Security/WindowsSignIn.aspx
AbsoluteURLReferrer:
UserAgent: Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 5.1; MAPSIE; .NET CLR 1.1.4322; .NET CLR 2.0.50727; InfoPath.1; .NET CLR 3.0.04506.30; MAPSIE; MAPSIE)
DefaultDataProvider: DotNetNuke.Data.SqlDataProvider, DotNetNuke.SqlDataProvider
ExceptionGUID: d9680c88-4202-41d6-947e-1172a2ae0778
InnerException: Could not load type 'DailyComic.DailyComicController'.
FileName:
FileLineNumber: 0
FileColumnNumber: 0
Method: System.Web.Compilation.BuildManager.GetType
StackTrace:
Message: System.Web.HttpException: Could not load type 'DailyComic.DailyComicController'. at System.Web.Compilation.BuildManager.GetType(String typeName, Boolean throwOnError, Boolean ignoreCase) at DotNetNuke.Framework.Reflection.CreateType(String TypeName, String CacheKey, Boolean UseCache, Boolean IgnoreErrors)
Source:

This one comes up during the first login attempt after the system has been rebooted or the application restarted.  Subsequent login attempts generate no errors...  I've re-installed the Daily Comic module, and have that one onilne again (and working), but those errors are still showing up.

I'll keep searching, but I'm fast running out of places to look.  The lack of error messages makes it difficult.  The only thing I can narrow it down to is something with 4.5.1.  I was able to go back and forth between 4.5.0 and 4.5.1 (until my backup got overwritten) and see the logins stop working every time I did the upgrade.  That "more-or-less" rules out configuration errors on the OS side.

Does anyone have any clues on what to test next?

I removed all the security roles that weren't being used anymore, and took a test user and removed them from all but the subscribers role.  The automated login still isn't working.

So, since automated logins ARE working for some people I can rule out most of the configuration settings.  Since everyone CAN log in via the Account Login module, I can rule out account problems (for the most part).  The problem seems to be that only a select number of users can use the automated login.  I took a test account that was not working, and set them identically as a student in DNN, and that didn't work.  That narrows it down to the AD account.

Where in the automated process would it make the distinction between different AD accounts?